1. Field of the Invention
The present invention relates to a magnetic hysteresis clutch or coupling device.
More particularly, it relates to a magnetic hysteresis clutch comprising an external rotor supporting a ferromagnetic hysteresis material and a rotary magnetic inductor arranged in the external rotor so as to define a radial air gap delimited on the outside by the ferromagnetic hysteresis material. The slip between the magnetic inductor and the ferromagnetic hysteresis material is varying between a minimum value (V1) and a maximum value (V2). The coupling between the magnetic inductor and the magnetic armature takes place by magnetization and demagnetization of the ferromagnetic hysteresis material which is in relative rotation with respect to the magnetic field.
These magnetic hysteresis clutches are used to transmit a torque from an input shaft of the clutch to an output shaft of the clutch in the case where there are significant differences between the speed of rotation of the input shaft and the speed of rotation of the output shaft of the clutch. A typical application of these magnetic hysteresis clutches is found in a reeling-in/reeling-out device delivering a reeling-in torque and a braking torque during reeling-out. In such a device, the input shaft of the clutch is coupled to the drive motor which has a nearly constant speed of rotation. The output shaft of the clutch is coupled to a reeling-in/reeling-out drum. The speed of rotation of the reeling-in/reeling-out drum passes through zero and changes direction, whereas the speed of rotation of the motor is substantially constant. If the modulus of the maximum speed of rotation of the output shaft of the clutch represents 50% of the modulus of the speed of rotation of the input shaft of the clutch, both during the reeling-out operation and during the reeling-in operation, the slip between the magnetic inductor and the magnetic armature of the clutch increases by 200% between the reeling-in operation and the reeling-out operation. This is because, during the reeling-in operation, the output shaft of the clutch rotates in the same direction as the input shaft, whereas, during the reeling-out operation, the output shaft of the clutch rotates in the opposite direction to that of the input shaft.
2. Prior Art
In a conventional magnetic hysteresis clutch, the torque transmitted increases with the slip between the magnetic inductor and the magnetic armature. This phenomenon is due to the fact that the magnetic field rotating with respect to the magnetic armature generates eddy currents in the latter which increase the coupling between the magnetic inductor and the magnetic armature. From patent application DE-A-2,139,009 it is known to amplify this phenomenon by fitting the magnetic armature with an element which is a good electrical conductor. This element is applied over the ferromagnetic material of the rotor. The transmitted torque of the magnetic clutch thus obtained strongly increases when the slip between the magnetic inductor and the magnetic armature increases.
European patent EP-A-0,269,535 discloses a magnetic hysteresis clutch wherein the transmitted torque increases only slightly with the slip between the magnetic armature and the magnetic inductor. In this clutch a ferromagnetic hysteresis material which is a poor electrical conductor is used for the magnetic inductor, thus leading to a decrease of eddy currents. More precisely, the ferromagnetic hysteresis material is a molded composite paste comprising a matrix based on a resin in which particles of a hysteresis magnetic material are dispersed. The working temperature of this composite material must be limited to 100.degree. C. so as to prevent its destruction.
For some applications it is however recommended to have a clutch with a negative characteristic curve, that is a clutch wherein the transmitted torque decreases when the slip increases.